1. Field of the Invention
The present invention relates to a digital information reproducing apparatus such as a magneto-optical disk reproducing apparatus.
2. Description of the Related Art
In a digital information storing apparatus for recording and reproducing or just reproducing digital information along tracks on a recording medium included in an optical disk unit, a magnetic disk unit and a magnetic tape unit, an amplitude detection or a peak detection is carried out for the purpose of converting a reproduction signal output from a reproduction head into a digital binary signal.
For example, the optical disk unit employs an amplitude detecting system or a peak detecting system according to a recording system of the digital data. The amplitude detecting system and the peak detecting system respectively correspond to an NRZI system and a RZ system. In the NRZI system, the information to be detected is a front edge and a rear edge of a recorded mark.
Hence, the reproduction signal is converted into a digital signal by binary-comparing the reproduction signal with a proper threshold value. In the RZ system, the center of a recorded mark is information to be detected. It is, therefore, necessary to differentiate the reproduction signal for seeking a zero cross point.
In this method, however, it is impossible to separate a signal pulse for the center of the recorded mark from a noise pulse generated on the noise of the reproduction signal. Under this method, a data error is more likely to take place. To overcome this shortcoming, even in a peak detecting system, it is necessary to detect an amplitude for distinguishing the signal from the noise according to the amplitude of the reproduction signal.
Herein, in the amplitude detecting system, it is necessary to transmit the reproduced waveform containing DC components. On the other hand, because of the problems such as low frequency variation due to reflectivity variation of a recording medium, occurrence of DC offset, and difficulty in arranging a wide-band amplifier, in a reproducing circuit system, it is necessary to take the steps of removing low frequency components from the reproduction signal and constantly controlling a threshold value of a comparator for converting the reproduction signal into a digital signal to be a center value of the reproduction signal according to the reproduction signal level. The prior art of such a reproduction circuit is shown in FIG. 13.
In FIG. 13, the reproduction signal converted into an electric signal through the effect of the reproduction head is amplified up to a proper amplitude level in an AC-coupled preamplifier 500 and then is guided to a low-pass filter 501. The low-pass filter 501 operates to remove unnecessary high-frequency components of the reproduction signal. The output of the low-pass filter 501 is guided to an equalizing circuit 502 for compensating for a restricted bandwidth caused according to resolution of the reproduction head. The output of the equalizing circuit 502 is guided to one input terminal of the comparator 503 and the first envelope detecting circuit 504 and the second envelope detecting circuit 505.
A first envelope detecting circuit 504 is provided for detecting a positive side envelope of the output of the equalizing circuit 502 and is arranged to have a diode 610, a resistor 611, a capacitor 612 and a buffer 613. A second envelope detecting circuit 505 is provided for detecting a negative side envelope of the output of the equalizing circuit 502 and is arranged to have a diode 620, a resistor 621, a capacitor 622 and a buffer 623.
The outputs of the envelope detecting circuits 504 and 505 are guided to an operating unit 506 in which the outputs are added to form one signal and then the resulting signal is subjected to filtering for removing a ripple. The output of the operating unit 506 is guided to the other input terminal of the comparator. According to the above arrangement, the reproduction signal is constantly controlled to keep the threshold value of the comparator for converting into a digital signal at the center of the reproduction signal according to the reproduction signal level.
In such a reproducing circuit, however, the envelope detecting circuit operates to follow even a disordered envelope of the reproduction signal caused by a minute defect on the recording medium. That is, the capacitors 612 and 622 store large charges corresponding to an amplitude of the reproduction signal obtained when the reproduction head passes along a minute defect, After it passes along the minute defect, the charges are discharged at a time constant defined by the combination of the capacitor 612 or 622 and the resistor 611 or 621. Hence, a considerably long time is taken in returning the charges to the original amplitude of the reproduction signal. During this period, it is impossible to obtain a proper threshold value for the reproduction signal level, thereby bringing about a reproduction data error. That is, even a minute defect causes the reproduction data error to be like a burst, thereby disadvantageously lowering reliability of the data.